1. Field of the Invention
The present invention relates generally to a micro-lens. More particularly, the present invention relates to a method of fabricating a micro-lens and a method of fabricating an optical module having a micro-lens integrated therein using the method.
2. Description of the Related Art
Micro-lenses are becoming increasingly popular today and can be found components such as optical sensors, optoelectronic products, and optical communications such as optical coupling between optical devices including an optical fiber, a PLC (Planar Lightwave Circuit), an LD (Laser Diode), and a PD (Photo Diode).
There have been many methods proposed that attempted to achieve high optical coupling efficiency in optical coupling between optical devices. Traditionally in the free space optic field, an optical device is assembled with a spherical lens ranging from hundreds of micrometers to tens of millimeters in diameter, a GRIN (Graded Index) lens; there can also be one or two aspherical lenses used, and a hemispherical-ended optical fiber is used to increase light receiving efficiency. Owing to the recent development of integrated optical devices, a micro-lens having a diameter of hundreds of micrometers or less and capable of collecting light perpendicularly to an Si, InP, or SiO2 substrate and a lens array having such micro-lenses are also formed.
Such a micro-lens array is widely used in a CCD (Charge Coupled Device) image sensor and an FPA (Focal Plane Array), especially in a Hartmann-Shack Wavefront sensor for measuring the aberrations of the eye. A major conventional method of fabricating such the micro-lens and lens array is photoresist (PR) reflow.
FIGS. 1A to 1D sequentially illustrate a prior art PR reflow-based micro-lens fabricating method, and FIG. 2 is an enlarged microscope picture of a micro-lens fabricated by the conventional PR reflow.
Referring to FIG. 1A, thick photoresist PR patterns 12 are formed on a substrate 11 of silicon, quartz, InP, or GaP.
Subsequently, as shown in FIG. 1B, the PR patterns 12 are shaped into convex lenses 13 due to surface tension by reflow at a high temperature.
After shaping the PR patterns into convex lenses 13, FIG. 1C shows the next step wherein the convex lens-shaped PR patterns 13 are transferred to the substrate 11 by an etching process using the PR patterns 13. One such type of etching process that can be used is, for example, RIE (Reactive Ion Etching).
Finally, at FIG. 1D, the lenses 14 are shown after fabrication is completed.
This conventional micro-lens fabricating method suffers from a low yield because the PR is very sensitive to temperature and humidity, making it difficult to easily reproduce the micro-lenses. Moreover, the thickness of the lenses is limited. Therefore, the micro-lenses are not feasible for light collection in parallel to the substrate with optical devices, such as an LD and an optical fiber/PLC for optical coupling, integrated thereon.
Referring to FIG. 3, in an attempt to solve the manufacturing problems, a non-reflective coated aspherical or spherical lens 31, which has a diameter ranging from hundreds of micrometers to tens of millimeters, is inserted between a laser diode LD 32 and an optical fiber/PLC 33.
However, the distance between the LD 32 and the spherical lens 31 is at least hundreds of micrometers. Thus, when light emitted from the LD 32 passes through the spherical lens 31, light that passes through the center of the spherical lens 31 and light that passes through the periphery of the spherical lens converge to different positions, resulting in a very low optical coupling efficiency, somewhere on order of about 10%. If an aspherical lens is used, the optical coupling efficiency will be increased, but the increased costs can be prohibitive in the practical use of such lenses for this purpose.
FIG. 4 illustrates a conventional method of construction, wherein an LD 41 formed on a substrate 41 by flip-chip bonding can be light-coupled directly to an optical fiber/PLC 43 within a range of tens of micrometers.
However, the conventional method illustrated in FIG. 4 suffers from a coupling loss due to the mode mismatch between the LD 42 and the optical fiber/PLC 43. Since the coupling loss is very sensitive to bonding accuracy, bonding errors cause a great amount of loss.